1. Field of the Invention
The present invention relates to phosphoric acid and its use, and more particularly the present invention relates to a process for the preparation of an analytical sample for use in quantitative analysis of a radioactive impurity in phosphoric acid; an analytical process for quantitatively analyzing a radioactive impurity in phosphoric acid; a process for the preparation of highly purified phosphoric acid substantially containing no radioactive impurity; and a process, for the production of semiconductor devices, in which highly purified phosphoric acid is used as a processing solution. Particularly, the present invention can be advantageously applied to a fabrication process, for semiconductor devices, in which phosphoric acid is used as a processing solution.
2. Description of the Related Art
It is well known that phosphoric acid is frequently used as an etching agent in the process of fabrication of semiconductor devices. For example, wet etching, in which phosphoric acid is used as a processing solution, is applied to selectively remove an insulating layer or film, e.g., a thin film of silicon nitride formed on a semiconductor substrate. However, phosphoric acid usually contains a radioactive nuclear species, e.g., .sup.210 Po (half-life period=138.4 days), as an impurity, which is apt to be either adsorbed on a silicon substrate in the phosphoric acid processing, or deposited thereon as a function of non-electrolytic plating. After finishing a series of steps for fabrication of a semiconductor device, .sup.210 Po, which was previously adsorbed on the silicon substrate, may emit .alpha.-ray, which leads to soft errors (memory errors) in a electronic device, e.g., a semiconductor memory, etc. The reason, why .sup.210 Po is contained in phosphoric acid, mainly resides in the composition of phosphorous ore which was used as the starting material for preparation of phosphoric acid. As described in K. Kubo, "Industrial Inorganic Chemistry", p.p. 121 to 129, published by Asakura Publishing Co. (October 5, 1962), phosphoric acid is produced either by the wet process, in which phosphoric acid is obtained by acidic decomposition of phosphoric ore with sulfuric acid, or by the dry process, in which phosphoric ore is reduced to phosphorus, which is, then, oxidized to phosphorous pentaoxide, which is finally, dissolved in water to obtain phosphoric acid. However, radioactive impurities contained in the raw phosphoric ore may still remain in the resulting phosphoric acid without being removed.
The soft errors of an electronic device are not only generated by .sup.210 Po set forth above, but also by the other radioactive impurities contained in phosphoric acid, according to the inventors' knowledge. The impurities which are expected to cause soft errors, are, for example, Bi (214, 210, 215, 212), Pb (214, 210, 211, 212), Ac (227, 228) and Th (234, 230, 231, 227, 232, 228). Particularly, the problems of Pb, Bi and Po are important. Therefore, the invention is expected to eliminate the undesirable influence caused by such radioactive impurities.
The mechanism of generation of soft errors will be more clearly understood after referring to the background technologies described hereinafter.
Japanese Unexamined Patent Publication (Kokai) No. 3-207596 points out that the existence of a very small amount of the radioactive elements .sup.210 Pb and .sup.210 Po contained in lead alloy solder material, is a cause of generating errors in the memory of semiconductor devices, e.g., ICs, LSIs etc., and teaches reduction of the content of such radioactive elements to less than 5 ppb to decrease the counted number of radioactive .alpha.-particles. In addition, it teaches, in the lower column on page 2, that .sup.210 Pb and .sup.210 Po emit .beta.-ray and .alpha.-ray as a result of the following radioactive disintegration:
.sup.210 Po.fwdarw.(.beta. disintegration).fwdarw..sup.210 Bi.fwdarw.(.beta. disintegration).fwdarw..sup.210 Po.fwdarw.(.alpha. disintegration).fwdarw..sup.206 Pb
Particularly, it is disclosed that .alpha.-disintegration, which is generated in the course of transformation of .sup.210 Po to .sup.206 Pb, is a cause of errors in memory.
Japanese Examined Patent Publication (Kokoku) No. 60-15152 teaches that a resin-sealed highly integrated semiconductor memory device, which does not suffer from soft errors due to .alpha.-ray, can be fabricated by providing an at least 40 .mu.m thick .alpha.-ray shielding layer which contains a resinous material contaminated with not more than 0.2 ppb of the total content of uranium and thorium, on the memory element. As described in "Kagaku Dai-jiten (Great Chemical Dictionary)", Vol. 8, pp. 811 to 812, published by Kyoritsu Publisher (Feb. 28, 1937), .sup.210 Po is a radioactive element having a half value period of 138.401 days, which Nr. and Mrs. Curie discovered, along with radium, in pitchblende. The quantitative analysis of .sup.210 Po is usually effected by determinating the intensity of .alpha.-ray emitted thereby, and its separation from the other elements can be effected by various processes, i.e., (1) deposition, (2) electrolytic deposition, (3) volatilization, (4) ion-exchange resin method and (5) solvent-extraction. Particularly, the process, in which, .sup.210 Po is electrolytically deposited on a metallic plate and evaporated in vacuum, is considered to be the most reliable.
Japanese Unexamined Patent Publication (Kokai) No. 55-48645 discloses a process for analysis of .alpha.-ray emitting-nuclear species contained in a waste liquid etc. discharged from an atomic power facility to the surrounding environment. This process comprises the following steps: An electrolytic deposition cell made of a resinous material, e.g., tetrafluoroethylene resin is equipped with a bottom plate which serves as a negative electrode plate for electrolytic deposition. A mixed liquid composed of a sample solution and an electrolytic solution is charged into the electrolytic deposition cell, and a positive electrode is dipped into this mixed liquid. And an electrical voltage is applied between the electrodes, while controlling the temperature of the mixed liquid by inserting a lower portion of the electrolytic deposition cell into a temperature-controlled medium. The .alpha.-ray emitting nuclear species contained in the sample solution is deposited on the negative electrode plate, and analyzed by applying the deposited electrode plate to an apparatus for measuring .alpha.-ray, e.g., a pulse height analyzer for .alpha.-ray measurement. It is also possible to effect such an analysis by adding a known amount of other .alpha.-ray emitting nuclear species, in order to improve the analytical accuracy or to effect correction of the measurement. However, according to the description, the undesirable effects of the other radioactive elements, e.g., .sup.210 Po etc. contained in phosphoric acid is not recognized, but also the analysis of such other elements is not discussed.
A number of technologies relating to the process for detecting radioactive elements, i.e., a process for recovering actinid series elements, including uranium from the waste of wet process of phosphoric acid, a process for regenerating phosphoric acid from its waste etc., have been disclosed. For example, U.S. Pat. No. 4,336,451 discloses preparation of a leaching acidic solution (nitric acid or hydrochloric acid etc.) in which radioactive elements of the uranium series and the thorium series are dissolved, and depositing or transferring these elements by means of non-electrolytic plating or electrolytic plating on a metallic substrate and, then, detecting the radioactive disintegration. However, this patent does not refer to the analysis of polonium in phosphoric acid. U.S. Pat. No. 3,983,219 discloses in the description of the prior art that it is possible to deposit .sup.210 Po on a platinum negative electrode from a 4 to 8N nitric acid solution by means of an electrolytic process.
Furthermore, U.S. Pat. No. 4,450,142, No. 4,162,230, No. 4,200,620 and No. 5,316,748 disclose processes for fabrication of highly purified phosphoric acid, and U.S. Pat. No. 4,749,455 and No. 4,615,776 disclose processes for regeneration of phosphoric acid waste.